Use the correct size when allocating the cmdbuf string.

cmdlengthdelta is the size of the header and we were using it to
allocate a buffer to store the command line. This would mean that
the cmdbuf could be too short. In practice this was never noticed unless
you usually run top -a. On a stock FreeBSD system you can see the
problem by running sendmail and then running top -a on a big terminal
window. In practice this doubles to size available to cmdbuf since the
header is around 65-68 bytes.

Reviewed by: adrian